The following abbreviations that appear in the ensuing description are defined as follows:
GSM: global system for mobile communications;
GPRS: general packet radio service;
MCH: multicast channel;
NTSC: national television system(s) committee;
OFDM: orthogonal frequency division multiplexing;
RTP: real-time transport protocol;
RTCP: RTP control protocol;
SCH: shared channel;
UMTS: universal mobile telecommunication system.
Multimedia broadcast/multicast service (MBMS) will provide the capability for data from a single source entity to be transmitted to multiple endpoints. MBMS is an internet protocol datacast (IPDC) service that can be offered via existing UMTS and GSM wireless networks. MBMS differs from broadcast in that the recipient/user of a broadcast signal need not be a subscriber; MBMS is a subscription based technology (whether paid or free) because the user must be a member of the multicast group to receive the MBMS signal. Further, MBMS provides a feedback channel for user interaction, whereas broadcast does not enable such functionality (though content in MBMS is still one-way only).
MBMS is a UMTS feature, which was finalized in June 2005 as 3GPP Release 6. Strong initial acceptance indicates that first networks, then individual user equipment UE, will support MBMS in the near term on an increasingly broad basis. MBMS technology preserves capacity and reduces costs by providing an efficient means to reliably distribute multimedia content over 3G networks. As a potential fee-based service, it also represents an unrealized potential revenue stream for wireless network operators, so much research has gone into developing MBMS.
A typical MBMS environment is illustrated in prior art FIG. 1. The content to be sent to a user may be provided by any of several sources 10, including an open service access point 12 or the Internet 14, and channeled through a broadcast/multicast service center BM-SC 16. It then routes through a gateway general packet radio service (GPRS) support node GGSN 18 and possibly also a backup camel server CSE 20 to a serving GPRS support node SGSN 22. From there the signal is sent to a wireless network, such as a UTRAN (UMTS terrestrial radio access network) 24 and/or a GERAN (GSM EDGE radio access network) 26, which provides the MBMS signal through its node B's or base transceiver stations (not separately shown in FIG. 1) to the various user equipment UE 28 within those networks under control of the node B's. A home location register HLR 30 may be checked by the SGSN 22 to determine which UEs 28 are in the multicast group. A cell broadcast center CBC 31 may also link the broadcast/multicast service center BM-SC 16 to the wireless networks 24, 26 to determine which UE's are entitled to receive additional data, for example, the enhancement channels described below.
Streaming applications (e.g. mobile digital TV) is anticipated to become a significant aspect in the long term evolution LTE (also known as 3.9G or E-UTRAN of the third generation partnership project 3GPP) of MBMS. Layered coding is a popular way of transmitting video streaming over the Internet to adapt to the changes of path delay, path bandwidth and path error on the Internet. Rate scalability of the streaming can be elegantly achieved by scalable video codecs that provide layered embedded bit-streams that are decodable at different bitrates, with gracefully degrading quality. Layered representations for Internet streaming have been widely studied. In addition, scalable representations have become part of established video coding standards, such as MPEG and ITU-T H.263 et seq. Scalable video representations aid in transport control protocol TCP-friendly streaming, as they provide a convenient way for performing the rate control required to mitigate network congestion. In receiver-driven layered multicasting, video layers are sent in different multicast groups, and rate control is performed individually by each receiver by subscribing to the appropriate groups. Layered video representations have further been proposed in combination with differentiated quality of service (DiffServ) in the Internet. The idea is to transmit the more important layers with better, but more expensive, quality of service (QoS), and the less important layers with fewer or no QoS guarantees.
For example, a scalable representation of video signals may consist of a base layer and multiple enhancement layers. The base layer provides a basic level of quality and can be decoded independently of the enhancement layers. On the other hand, the enhancement layers serve only to refine the base layer quality and alone are not useful. Therefore, the base layer represents the most critical part of the scalable representation, which makes the performance of streaming applications that employ layered representations sensitive to losses of base layer packets.
Further background detail concerning base and enhancement layers may be found at International Publication No. WO 2005/039186 published on Apr. 28, 2005 and entitled SCALEABLE ENCODING FOR MULTICAST BROADCAST MULTIMEDIA SERVICES, by Lorenzo Casaccia et al. Briefly, that document is seen to describe splitting MBMS content into a base layer (e.g., video in low quality/resolution and only a black and white color scheme) and one or more enhancement layers (e.g., data for increased quality/resolution and color).
Digital TV is considered a service for LTE MBMS with potentially wide adoption potential, but it requires a very large bandwidth. For example, one MPEG2 HDTV (motion picture expert group 2, high definition television) streaming video needs 15-20M bps (million bits per second). It is technically and economically inefficient to transmit the whole HDTV streaming signal over dedicated MBMS channels.
Further, MBMS single frequency networks require synchronization between MBMS Node Bs (e-Node Bs in E-UTRAN). If MBMS functionality is integrated into existing unicast e-Node Bs, it will result in a fully synchronized radio access network. This is seen as a distinct disadvantage because full synchronization would restrict individual cells in the network from allocating their radio resources (e.g., bandwidth, spreading codes, temporary identifiers, etc.) as freely as they do now, resulting in overall decreased efficiency. Additionally, LTE tends toward an asynchronous mode for the e-Node Bs operating for unicast services.
There are several constraints to keep in mind when developing MBMS. The relevant 3GPP technical specification TS 25.813 stipulates that the E-UTRA/E-UTRAN (where the prefix E represents “evolved”) network permit simultaneous, tightly integrated, and efficient provisioning of dedicated (e.g., unicast) and MBMS services to the user; that MBMS transmissions from several e-Node B's may be coordinated; and that MBMS may be provided on a frequency layer dedicated to MBMS as well as on a frequency layer shared with non-MBMS services. The frequency layer dedicated to MBMS is to be a set of cells dedicated to MBMS, whereas the frequency layer shared with non-MBMS services is to be a set of cells supporting both unicast and MBMS services. Coordination of MBMS transmissions within a single frequency network SFN may be done among several e-Node B's of that same SFN area, and SFN's may be differently defined in multiple SFN areas.
What is needed in the art is a way to efficiently use available bandwidth for multimedia transmissions (e.g., base and enhancement layers) while not overly burdening the system doing the transmitting by requiring tight synchronization across the entire multimedia transmissions and/or inherently restricting the network's flexibility in executing its other functions such as handling uplink and downlink user data (e.g., regular wireless phone calls or exchange of messages).